1. Field of the Invention
The present invention relates to apparatuses and methods for implementing optical differential phase shift keying (DPSK) demodulation in a communication system, and particularly for implementing optical DPSK demodulation in a satellite communication system.
2. Description of the Related Art
Optical DPSK modulation schemes generally allow a 3 dB signal gain over conventional on-off keyed systems, which make them ideal for satellite communications. Optical DPSK receivers require a demodulator, typically a Mach-Zhender interferometer as is known in the art, to convert the bit stream from phase-shift keyed to on-off Keyed.
The efficiency of an optical DPSK communication link can be characterized by its bit-error-rate (BER). The BER is greatly affected by the quality of the demodulator. In fact, in many applications a very pure optical interference is required to achieve acceptable BER. This purity can be measured by the extinction ratio (ER). Typically in the art, a fiber Mach-Zhender interferometer is used as a DPSK demodulator because it has low loss and is fairly robust. On the other hand, the optical state of polarization (SOP) is randomly changed within the fiber by macro and micro bends, random stresses, inherent imperfections, and thermal and accoustic perturbations. Because these SOPs are randomly scrambled, optical interference is degraded which dramatically degrades the ER.
In view of the foregoing discussion, there is a need in the art for an interferometer which is robust, precise and easily manufactured. In addition, there is particularly a need for such an interferometer in an optical differential phase shift keying (DPSK) demodulator in a communication system, such as a satellite communication system. As detailed hereafter, the present invention meets these needs.